1. Field of the Invention
This invention relates to a device for producing a high frequency modulation magnetic field that is used for applying a modulating magnetic field in a magneto-optical recording and/or reproducing device. More particularly, it relates to such device allowing real time overwriting of recording signals in the high frequency range.
2. Prior Art
Magneto-optical recording is the recording system in which information signals are written on an magneto-optical recording medium such as an magneto-optical disk, by application of heat generated by a laser light. The magnetization direction is determined by a magnetic field applied to the heated portion upon cooling.
One of the features of the magnetic-optical recording is that it is possible to re-write the information signals. Thus, in the magneto-optical recording system, it is possible to write new data in an area of the magneto-optical recording medium where a recording pattern was written previously.
Such re-writing is effected by inverting the bias magnetic field. For providing the strong bias magnetic field having more than a predetermined magnetic field intensity, it is known to use a permanent magnet for excitation of the magnetic field. In this method, the permanent magnet is moved by mechanical means for inverting the direction of the magnetic field, and the recording and erasure processes are provided separately. For example, the magnet is mechanically inverted in its position after the first erasure for recording on the area on the magnetic recording medium from which the recording has just been erased.
It is also known to use distinct erasure and recording devices that are adapted to take charge of different portions of the overwrite function. In this known method, erasure and recording are effected at the different portions on the optomagnetic recording medium. However, real-time recording is difficult to achieve with the above described methods.
With the first method of using the permanent magnet, high-speed operation is not realized with ease because of unavoidable limitations placed on the operating speed. With the method of using the distinct erasure and recording devices, since the devices are located at the spatially separate positions, it is not possible to achieve a completely consecutive recording especially in the case of the magneto-optical recording with a high recording density.
On the other hand, real-time overwriting can be achieved by using a magnetic field modulation system as shown in FIG. 10. In this system, there are provided a laser device 91 for heating an magneto-optical recording medium 90 by a laser beam through a lens 93 and a magnetic field producing device 92 for producing the magnetic field modulated by input signals.
However, when a high frequency current such as several MHz are supplied to the magnetic field producing device 92 in order to meet the demand for high speed operation or improvement in the recording density, it is not possible to attain the desired magnetic field intensity, while there is also an inconvenience of heat generation.
Thus, in the magnetic field modulation system, it is necessary to apply a magnetic field of a high constant intensity in order to invert the direction of magnetization of the magneto-optical recording medium 90. However, when the high frequency signals are supplied, it becomes difficult to achieve the desired strong magnetic field because of frequency characteristics of the core material of the magnetic field producing device 92. Conversely, it is possible to produce a magnetic field sufficient to invert the magnetization direction by increasing the signal current supplied to the magnetic field producing device 90. However, in this case, problems are presented when the core characteristics are deteriorated due to heat generation or when the temperature becomes higher than the Curie temperature. As a result, it becomes impossible to produce the strong magnetic field required in the high frequency range. In addition, a stable recording operation is sometimes impossible to achieve because the heating performance of the laser beam is affected by the heat produced from the magnetic field producing device 90.